Cutting-edge near-infrared luminescent lanthanide(III) complexes: from molecular design to emerging applications

Near-infrared (NIR) luminescent lanthanide(III) (Ln III ) complexes have emerged as attractive molecular materials for bioimaging, sensing, and multifunctional photonic applications. Their narrow and characteristic emission bands, long excited-level lifetimes (10 −6 –10 −3 s), and access to biologically and technologically relevant spectral regions provide advantages over conventional organic fluorophores. Nevertheless, efficient NIR luminescence remains challenging because the low-lying 4 f excited levels are particularly susceptible to multiphonon relaxation, while the weak 4 f –4 f absorption transitions require efficient sensitization. This review provides a comprehensive assessment of the molecular strategies developed to overcome these limitations. We first discuss the electronic structure and photophysical properties of NIR-emitting Ln III ions, followed by the main ligand-to-Ln III energy transfer mechanisms, including phonon-assisted processes, photoinduced electron transfer, ligand-to-metal charge-transfer-mediated pathways, and cooperative energy transfer. We then examine how ligand design and coordination architectures, including β-diketonates, quinolinates, porphyrins, cryptates, metallacrowns, and d–f assemblies, control vibrational quenching, excited-level dynamics, and emission efficiency. Finally, emerging applications in NIR bioimaging, lifetime imaging, multiplexing, circularly polarized luminescence, and magneto-luminescence are discussed, highlighting the structure-photophysics relationships that underpin the development of next-generation NIR Ln III complexes.